Numerous muffler constructions have been proposed for the attenuation of the sound component of an exhaust gas stream from an internal combustion engine. Invariably, these structures have purported to effect sound attenuation without substantially or intolerably increasing the back pressure on the engine. As is well known, muffler induced back pressure will substantially reduce internal combustion engine performance. The problem of reduced performance is most extreme in high-performance racing engines. The "solution" to the problem which is actually used usually is merely to employ a straight pipe from the engine and tolerate the noise. With urban expansion, however, even race tracks are under pressure to reduce the noise level during racing. Moreover, at least some high performance cars also are driven, at least occasionally, on the city streets. In order to be "street-legal" such high performance engines must be coupled to a muffler, and the only mufflers which are currently commercially available that are used on such high-performance engines cause a significant drop in engine power as a direct result of the back pressure induced in the muffler.
Typically, a 575 horsepower engine will produce a noise level of about 130 db at hard acceleration with no muffler, and on the same engine when a commercially available high-performance muffler is used, the noise level will be reduced to about 95 db (A scale) at hard acceleration, but there also will be an 18 to 20% power loss. Even larger engines, for example 700 to 800 horsepower, have more cam overlap and cannot tolerate sound attenuation to 95 db since it would produce a 30 to 40% power loss.
Another problem that complicates any attempt to attenuate sound in high-performance internal combustion engines is the necessity to minimize bulk and weight. The exhaust pipe on a high horsepower engine typically will be about 4 inches in diameter so as to accommodate the very substantial volumetric flow. Mufflers which depend upon excessive length or diameter to achieve sound attenuation will be unsuitable for use on race cars, either because of their bulk or weight, or both.
The patent art contains various muffler constructions which purport to solve the problem of sound attenuation without undesirable back pressure, but in fact these various structures have substantial performance deficiencies. It is well known to provide a divergently tapered centrally located conical partition for flow of gases around the partition to effect an expansion of the gases. Typical of such structures are the devices shown in U.S. Pat. Nos. 2,071,351, 2,239,549 and 2,971,599.
Some of these patented mufflers follow such an expansion partition or cone with a contraction or concentrating partition or baffle. Typical of such devices are the mufflers shown in U.S. Pat. Nos. 1,081,348, 2,667,940, 3,029,895 and 3,029,896. These mufflers, however, do significantly increase back pressure by causing the exhaust gases to reverse the direction of their flow axially as they attempt to pass beyond the concentrating or converging baffle. This flow reversal may be effective in sound attenuation, but it has been found to increase back pressure undesirably.
Even mufflers which employ alternating divergent and then convergent partitions have suffered from undesirable bulk and/or weight, inordinate complexity, or auxiliary flow channels or openings in the partitions which defeat sound attenuation. Typical of such mufflers are the mufflers set forth in U.S. Pat. Nos. 624,062, 1,184,431, 2,325,905 and 2,485,555.
Additional patent art known to applicant but believed to be peripheral in relevance to the present invention are the following U.S. Pat. Nos. 1,677,570, 1,756,916, 1,946,908, 2,934,889, 3,219,141, 3,786,896, 4,143,739 and 4,346,783.
The reality of the industry is that high-performance racing cars are either using no muffler or mufflers which barely achieve the desired sound attenuation, and achieve it at a significant power loss and with an undesirable increase in bulk and weight.